The best thing to happen to couponing is the internet. You can find coupons for just about anything! My favorite sites are coupons.com, coolsavings.com, redplum.com, and smartsource.com
These sites usually carry almost identical coupons, which makes it great for doubling up so if you do find a good sale, you've got multiple coupons to make it a better deal and stock up :)
What I have been trying to do is to look at the stores weekly adds that come out in the Sunday paper and match up coupons. If I don't have a coupon for something that is on sale that I could really use, I have ordered coupons. The only place I have bought coupons from so far has been e-Bay and have had really good luck with it. Paying for the coupons by Sunday night I have always had my coupons before the end of the week to use on the store sale. A word of caution though...people do seem to get carried away and some will over pay for their coupons, making them not worth it. So do your math before bidding, make sure you don't overpay. Also with EVERY auction on e-Bay, not just with coupons, look at the sellers feedback score. If it's below 99%, I will read their feedback to see what the problem is, most often it's a disgruntled buyer, but always shop with discretion.
Another site that sells coupons is Coupons and Things at Coupondede.com I have not tried this out yet, but looks really good. Over at Coupon-MakerOver.com, they have a coupon tom that you can type in what you're looking for and it will tell you if and where you can find those coupons. I'm not sure how I found the coupon tom in there so the direct link is http://couponmakeover.coupontom.com
And lastly, going directly to the manufactures website will sometimes get some good coupons. I do have two email address's and use one of them solely for getting newsletters from the companies I buy products to. You can also like these companies on Facebook and will often times give fans a heads up when a coupon will be out in the Sunday paper or a deal just for fans. For instance, Bounty paper towel tomorrow only (June 3) from 11-1 pm eastern time (7-9 am our time) they will be giving away 5,000 $5 coupons!
Another good example of getting newsletters direct from the companies is Payless sent me a coupon for 20% off my ENTIRE purchase that expires June 8th and they just started a BOGO sale today :)
So if you don't have a junk email, start one! Unless you like lots of junk mail in your regular email.